The invention relates to a receiver for a digital transmission system comprising a transmission channel having a storage depth n with an analog signal processing section; and more particularly to such a receiver which comprises an analog-to-digital converter and an equalizer which has a digital signal processing section that includes a memory means with a register for storing state transitions.
Such receivers are necessary for the future pan-European mobile radio system (GSM system). In this mobile radio system speech signals are transmitted in digitized form together with other digital signals according to a time-division multiple-access method. These data are transmitted by a transmitter of the mobile radio system via a suitable modulation. As a result of reflections and multipath propagation on the transmit path the transmitted signal reaches the receiver in various superposed signal portions presenting different delays and phase shifts. This distorts the signal transmitted by the transmitter. These distortions cause the bits contained in the received signal to be affected by preceding bits (intersymbol interference). Between transmitter and receiver there is a transmission channel having a storage depth n, where n is an integer and denotes the number of interfering neighbouring bits. For recovering the original data content of the signal, it is necessary to equalize the received signal.
DE-A 39 11 999 to which U.S. Pat. No. 5,119,400 corresponds discloses a receiver for a digital transmission system of the type described above. The receiver comprises an equalizer for forming binary estimates on the basis of a sequence of signal sample values. Each sample value can be assigned with time to a transmitted bit having the binary "0" or "1" value and depends on this bit and also on n immediately preceding bits. The equalization is then performed with the aid of the so-called Viterbi algorithm. According to this algorithm 2.sup.n states and a probability parameter for each transition from one state to a time-consecutive state are assigned to each sample value. By stringing the state transitions together, paths are formed and an overall probability parameter is formed from all the probability parameters of one path. From all the paths leading to one state only the path having the smallest overall probability is taken into consideration. A transition from a state determined by the bits b.sub.i-1 . . . b.sub.i-n at a discrete instant i with a binary value b.sub.i-n =0 (="0"-transition) to a next state determined by the bits b.sub.i . . . b.sub.i-n+1 at an instant i+1 determines so-called "0"-paths; and with a binary value b.sub.i-n =1 (="1"-transition) determines so-called "1"-paths. From the overall probability parameters of all the possible "0"-paths and from the overall probability parameters of all the possible "1"-paths, always the smallest overall probability parameter is selected. The path defined by the bit sequence b.sub.i, . . . b.sub.i-n for b.sub.i-n =0 or b.sub.i-n =1 respectively, and having the smallest overall probability parameter of all the 2.sup.n possible "0"-paths or "1"-paths characterizes the minimum "0"-path or minimum "1"-path respectively. The binary number, i.e. zero or one, assigned to the smaller of the two selected overall probability parameters denotes the estimate, while reliability information for this estimate is formed on the basis of the two selected overall probability parameters. The reliability information is estimated in a decoder following the equalizer in the system. The costs of realizing the equalizer are in a first approximation proportional to the number of states 2.sup.n, i.e. they exponentially increase with the storage depth n of the transmit channel.